Technical Abstract:
The rootstock phase of fire blight is a serious threat to apple orchards planted at high density on susceptible rootstocks, especially M.9 and M.26. A fire blight infection in these rootstocks will usually kill trees by girdling the rootstock. A single episode of 10% rootstock blight can result in losses of $1,500 to $3,500 per acre when the costs of tree replacement, lost investment in tree maintenance, and reduced yields over several years are considered. This article describes the biology of rootstock infection and its management. Rootstocks can become infected by internal spread of fire blight bacteria from infections in the scion, by infection of rootstock suckers (vegetative shoots developing from the rootstock), or by infection through wounds in rootstock bark caused by various injuries including boring insects. It is often recommended that fire blight infections be pruned out of young apple trees during the growing season to prevent rootstock infection. Pruning out scion infections 3 weeks after blossom inoculation and then repeatedly during the growing season gave variable results in 3 years of experiments. Pruning had no beneficial effect on preventing eventual death of the rootstock in 2 test seasons, but pruning reduced rootstock death in one of three years of testing. However, 2 to 3 consecutive years of pruning reduced fruit yield. Economic analysis indicated that pruning out fire blight infections did reduce the magnitude of financial loss, primarily due to reduced tree loss, but was not effective in managing this destructive phase of fire blight. New fire blight resistant apple rootstocks developed by the USDA-ARS/Cornell University apple rootstock-breeding program have potential to provide practical control for the rootstock phase of fire blight.